In GrantHub you aren't able to delete a user, but you can move them to Inactive.
Go to the top right-hand corner and click on the blue box that says, "Welcome, Your Name".
Click on My Organization from the drop-down.
Click on the Users tab.
Then select the user you would like to move to Inactive.
From the Status drop-down, move to Inactive and then in the Re-assign to User drop-down, select the new user to own the Opportunities, Tasks, Documents, and Answers.